Rolling code security system

ABSTRACT

A rolling code transmitter is useful in a security system for providing secure encrypted RF transmission comprising an interleaved trinary bit fixed code and rolling code. A receiver demodulates the encrypted RF transmission and recovers the fixed code and rolling code. Upon comparison of the fixed and rolling codes with stored codes and determining that the signal has emanated from an authorized transmitter, a signal is generated to actuate an electric motor to open or close a movable barrier.

This is a division, of prior application Ser. No. 08/873,149, filed Jun.11, 1997, which was a continuation of application Ser. No. 08/446,886,filed May 17, 1995, now abandoned, which is hereby incorporated hereinby reference in its entirety.

BACKGROUND OF THE INVENTION

The invention relates in general to security systems which allowoperation upon the receipt of a properly coded signal. Moreparticularly, the invention relates to a security system or to a barrieroperator system, such as a garage door operator, employing a transmitterand a receiver which communicate via code streams having at least aportion thereof which changes with multiple operation of the device.

It is well known in the art to provide garage door operators or otherbarrier operators which include an electric motor connectable through atransmission to a door or other movable barrier which is to be openedand closed. Since many of these systems are associated with residences,as well as with garages, it is important that opening of the barrier bepermitted only by one who is authorized to obtain entry to the areawhich the barrier protects. Some garage door operator systems have inthe past employed mechanical lock and key arrangements associated withelectrical switches mounted on the outside of the garage. While thesesystems enjoy a relatively high level of security, they are veryinconvenient to use for a person because it necessitates them exitingtheir vehicle in order to send the command to open the garage door. Thisalso may present some danger to people when the exit the relativesecurity of their vehicle if someone may be waiting to do injury tothem.

It is also well known to provide radio-controlled garage door operatorswhich include a garage door operator unit having a radio receiver and amotor connected to be driven from the radio receiver. The radio receiveris adapted to receive radio frequency signals or other electromagneticsignals having particular signal characteristics which, when received,cause the door to be opened. More recently, such transmitter andreceiver systems have become relatively more sophisticated in that theyuse radio transmitters which employ coded transmissions of multiple orthree-valued digits, also known as “trinary bits” or other serial codedtransmission techniques. Among these systems are U.S. Pat. No. 3,906,348to Willmott, which employs a transmitter and receiver system wherein aplurality of mechanical switches may be used to set a storedauthorization code.

U.S. Pat. No. 4,529,980 to Liotine et al. discloses a transmitter andreceiver combination for use in a device such as a garage door operatorwherein the transmitter stores an authorization code which is to betransmitted to and received by the receiver via a radio frequency link.In order to alter or update the authorization code contained within thetransmitter, the receiver is equipped with a programming signaltransmitter or light emitting diode which can send a digitized opticalsignal back to the transmitter where it is stored. Other systems alsoemploying encoded transmissions are U.S. Pat. Nos. 4,037,201, 4,535,333,4,638,433, 4,750,118 and 4,988,992.

While each of these devices have provided good security for the user, itis apparent that persons wishing to commit property or person-relatedcrimes have become more sophisticated as well. It is known in thesecurity industry today that devices are being made available that canintercept or steal rolling code.

Transequatorial Technology, Inc. sells integrated circuit code hoppingencoders identified as Keeloq Model NTQ105, NTQ115, NTQ125D and NTQ129.Some of the keeloq code hopping encoders generate serial codes havingfixed portions, i.e., which do not change with repeated actuation of theencoding portion of the chip and rolling code portions which alter witheach actuation of the encoding portion of the chip. In order to avoid,however, having the problem of the encoding portion of the chip havingbeen inadvertently enabled and causing the rolling code to be altered onsuccessive enabling attempts thereby leading to a rolling code which istransmitted and not recognized by a receiver, the keeloq code hoppingencoders provide a window forward system, that is they are operable withsystems having code receivers which recognize as a valid code not asingle rolling code, but a plurality of rolling codes within a certaincode window or window of values which are the values which would begenerated on a relatively small number of switch closures as compared tothe total number of rolling codes available. The problem with such asystem, however, might arise if a user was away for a period of time orhad inadvertently caused codes to be transmitted excluding the number ofcodes normally allowed within the valid forward code window. In thatcase, the rolling code would not be recognized by the receiver and theuser could not gain entry without taking other measures to defeat thelocking system or the garage door operator system which might involvethe intervention of a trained engineer or technician.

Texas Instruments also has a prior system identified as the Mark StarTRC1300 and TRC1315 remote control transmitter/receiver combination. Thesystem involves the use of a rolling code encoder which increments orrolls potentially the entire code, that is it does not leave a fixedportion. The system also includes a forward windowing function whichallows an authorized user to be able to cause the receiver to be enabledwithin a limited number of key pushes. Like the keeloq system, if theforward window is exceeded, the Texas Instruments system must be placedin a learn mode to cause the system to relearn the code. In order toplace the system into the learn mode, the person must obtain directaccess to the receiver to cause a programming control system associatedwith the receiver to be hand actuated causing the receiver to enter alearn mode. Once the receiver has learned the new code, the receiverwill then construct a new valid forward code window within which validrolling codes may be received. The problem, of course, with such asystem is that if, for instance in a garage door operator, the onlyportal of entry to the garage door is through the overhead doorcontrolled by the garage door operator, the user will not be able toobtain entry to the garage without possibly having to do some damage tothe structure. This problem is sometimes referred to in the industry asa “vaulted garage.”

What is needed is an economical encoding system which provides goodsecurity by using a rolling code, but which enables a user of the systemto proceed via a gradually degraded pathway in the event that thereceiver detects a signal condition indicative of what might be a lackof security.

SUMMARY OF THE INVENTION

The invention relates in general to an electronic system for providingremote security for entry of actuation of a particular device. Such asystem may include a transmitter and receiver set, for instance with ahand-held transmitter and a receiver associated with a vehicle such asan automobile or the like. The transmitter, upon signaling the receiver,causing the vehicle to start up or to perform other functions. Thesystem may also be useful in a barrier operator system such as a garagedoor operator by allowing the garage door to be opened and closed in arelatively secure fashion while preventing persons who may beintercepting the radio frequency signals from being able to, althoughunauthorized, cause the vehicle to begin running or to allow access tothe garage.

The system includes a transmitter generally having means for developinga fixed code and a rolling or variable code. The rolling or variablecode is changed with each actuation of the transmitter. The fixed coderemains the same for each actuation of the transmitter. In the presentsystem, the transmitter includes means for producing a 32-bit framecomprising the fixed portion of the code and a second 32-bit framecomprising the variable portion of the code. The 32-bit rolling code isthen mirrored to provide a 32-bit mirrored rolling code. The 32-bitmirrored rolling code then has its most significant bit “deleted” bysetting it to zero. The transmitter then converts the 32-bit fixed codeand the mirrored variable code to a three-valued or trinary bit fixedcode and a three-valued or trinary bit variable code or rolling code.

To provide further security, the fixed code and the rolling codes areshuffled so that alternating trinary bits are comprised of a fixed codebit and a rolling code bit to yield a total of 40 trinary bits. The 40trinary bits are then packaged in a first 20-trinary bit frame and asecond 20-trinary bit frame which have proceeding them a singlesynchronization and/or identification pulse indicating the start of theframe and whether it is the first frame or the second frame. Immediatelyfollowing each of the frames, the transmitter is placed into a quietingcondition to maintain the average power of the transmitter over atypical 100 millisecond interval within legal limits promulgated by theUnited States Federal Communications Commission. The first trinary frameand the second trinary frame are used to modulate a radio frequencycarrier, in this case via amplitude modulation to produce an amplitudemodulated encrypted signal. In a preferred embodiment, the radiofrequency signal is amplitude modulated. The amplitude modulated signalis then launched and may be received by an AM receiver. In the preferredembodiment, the AM receiver receives the amplitude modulated signal,demodulates it to produce a pair of trinary bit encoded frames. Thetrinary bits in each of the frames are converted on the fly to 2-bit orhalf nibbles indicative of the values of the trinary bits which areultimately used to form two 16-bit fixed code words and two 16-bitvariable code words. The two 16-bit fixed code words are used as apointer to identify the location of a previously stored rolling codevalue within the receiver. The two 16-bit rolling code words areconcatenated by taking the 16-bit word having the more significant bits,multiplying it by 3¹⁰ and then adding it to the second of the words toproduce a 32-bit encrypted rolling code. In order to make certain thatif the transmitter was inadvertently actuated a number of times, theauthorized user can still start his car or gain entry to his garage. The32-bit encrypted code is then compared via a binary subtraction with thestored rolling code. If the 32-bit code is within a window or fixedcount, in the present embodiment 1000, the microprocessor produces anauthorization signal which is then responded to by other portions of thecircuit to cause the garage door to open or close as commanded. In theevent that the code is greater than the stored rolling code, plus 1000,indicative of a relatively large number of incrementations, the user isnot locked out of the garage, but is allowed to provide further signalsor indicia to the receiver that he is an authorized user without anysignificant degradation of the security. This is done by the receiverentering an alternate mode requiring two or more successive valid codesto be received, rather than just one. If the two or more successivevalid codes are received, the garage door will open. However, in orderto prevent a person who has previously or recently recorded a recentvalid code from being able to obtain access to the garage, a trailingwindow, in this case starting at a count of 300 less than the presentstored count and including all code values between the present storedcount and 300 less is compared to the received code. If the receivedcode is within this backward window, the response of the system simplyis to take no further action, nor to provide authorization during thatcode cycle on the assumption that the code has been purloined.

Thus, the present system provides important advantages over the previousgarage door operator systems and even previous rolling code systems. Thesystem provides a multiple segmented windowed system which provides avalid code window, a second relatively insecure code window in which twosuccessive valid codes must be received and finally a window in which novalid codes are recognized due to the likelihood of the receiver havingbeen stolen.

It is a principal object of the present invention to provide a securitysystem involving a radio frequency transmitter and receiver whereinmultiple security conditions may exist requiring different levels ofsignal security.

It is another object of the present invention to provide a secure radiotransmitter receiver system which may rapidly and easily decode arelatively large code combination.

Other advantages of the invention will become obvious to one of ordinaryskill in the art upon a perusal of the following specification andclaims in light of the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an apparatus for moving a barrier orgarage embodying the present invention;

FIG. 2 is a block diagram of a transmitter for use with a garage dooroperator of FIG. 1;

FIG. 3 is a block diagram of a receiver positioned within a head unit ofthe garage door operator shown in FIG. 1;

FIG. 4 is a schematic diagram of the transmitter shown in FIG. 2;

FIGS. 5A-5B is a schematic diagram of the receiver shown in FIG. 3;

FIG. 6 is a timing diagram of signals generated by a portion of thetransmitter;

FIGS. 7A, B and C are flow diagrams showing the operation of thetransmitter; and

FIGS. 8A, B, C, D, E and F are flow charts showing the operation of thereceiver.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings and especially to FIG. 1, morespecifically a movable barrier door operator or garage door operator isgenerally shown therein and includes a head unit 12 mounted within agarage 14. More specifically, the head unit 12 is mounted to the ceilingof the garage 14 and includes a rail 18 extending therefrom with areleasable trolley 20 attached having an arm 22 extending to a multiplepaneled garage door 24 positioned for movement along a pair of doorrails 26 and 28. The system includes a hand-held transmitter unit 30adapted to send signals to an antenna 32 positioned on the head unit 12and coupled to a receiver as will appear hereinafter. An externalcontrol pad 34 is positioned on the outside of the garage having aplurality of buttons thereon and communicate via radio frequencytransmission with the antenna 32 of the head unit 12.

An optical emitter 42 is connected via a power and signal line 44 to thehead unit. An optical detector 46 is connected via a wire 48 to the headunit 12.

Referring now to FIG. 2, the transmitter 30 is shown therein in generaland includes a battery 70 connected by a pushbutton switch 72 to a powersupply 74 which is coupled via leads 75 and 76 to a microcontroller 78.The microcontroller 78 is connected by a serial bus 79 to a non-volatilememory 80. An output bus 81 connects the microcontroller to a radiofrequency oscillator 82. The microcontroller 78 produces coded signalswhen the button 72 is pushed causing the output of the RF oscillator 82to be amplitude modulated to supply a radio frequency signal at anantenna 83 connected thereto. More specifically, as shown in FIG. 4,details of the transmitter 30 are showntherein, including a plurality ofswitches 72. When switch 72 is closed, power is supplied through a diode100 to a capacitor 102 to supply a 7.1 volt voltage at a lead 103connected thereto. A light emitting diode 104 indicates that thetransmitter button has been pushed and provides a voltage to a lead 105connected thereto. A Zener diode 106 provides voltage regulation andcauses the back biased diode 107 to cause the crystal 108 to beenergized, thereby energizing the microcontroller 78, a Zilog 125C01138-bit microcontroller in this embodiment. The signal is also sent via aresistor 110 through a lead 111 to a P32 pin of the microcontroller 78.Likewise, when a switch 113 is closed, current is fed through a diode114 to the lead 103 also causing the crystal 108 to be energized,powering up the microcontroller at the same time that pin P33 of themicrocontroller is pulled up. Similarly, when a switch 118 is closed,power is fed through a diode 119 to the crystal 108 as well as pull upvoltage being provided through a resistor 120 to the pin P31. It shouldalso be appreciated that pin P34 of the microcontroller is configuredvia a connection with the resistor 123 to be an RS232 input port 124.

The microcontroller is coupled via the serial bus 79 to a chip selectport, a clock port and a DI port to which and from which serial data maybe written and read and to which addresses may be applied. As will beseen hereinafter in the operation of the microcontroller, themicrocontroller 78 produces output signals at the lead 81, which aresupplied to a resistor 125 which is coupled to a voltage dividingresistor 126 feeding signals to the lead 127. A 30-nanohenry inductor128 is coupled to an NPN transistor 129 at its base-130. The transistor129 has a collector 131 and an emitter 132. The collector 131 isconnected to the antenna 83 which, in this case, comprises a printedcircuit board, loop antenna having an inductance of 25-nanohenries,comprising a portion of the tank circuit with a capacitor 133, avariable capacitor 134 for tuning, a capacitor 135 an a capacitor 136.An 30-nanohenry inductor 138 is coupled via a capacitor 139 to ground.The capacitor has a resistor 140 connected in parallel with it toground. When the output from lead 81 is driven high by themicrocontroller, the transistor Q1 is switched on causing the tankcircuit to output a signal on the antenna 83. When the transistor Q1 isswitched off, the output to the drive the tank circuit is extinguishedcausing the radio frequency signal at the antenna 83 also to beextinguished.

Referring now to FIG. 3, the receiver is shown therein and includes areceiver antenna 200 coupled to an amplitude modulated receiver 202driven from a power supply 204 connectable to a source of alternatingcurrent 206. The receiver 202 provides a demodulated output via abandpass filter 210 to an analog-to-digital converter 212 which providesinput to a microcontroller 214 having an internal read-only memory 216and an-internal random-access memory 218. A serial non-volatile memory220 is connected via a memory bus 222 to the microcontroller 214 to sendand receive information thereto. The microcontroller has an output line226 coupled to a motor controller 228 which may include a plurality ofrelays or other standard electromechanical features which feedselectrical current on lines 230 and 232 to an electric motor 234.

Referring now to FIGS. 5A-5B the antenna 200 coupled to a reactivedivider network 250 comprised of a pair of series connected inductances252 and 254 and capacitors 256 and 258 which supply an RF signal to abuffer amplifier having an NPN transistor 260, at its emitter 261. TheNPN transistor 260 has a pair of capacitors 262 and 264 connected to itfor power supply isolation. The buffer amplifier provides a bufferedradio frequency output signal on a lead 268. The buffered RF signal isfed to an input 270 which forms part of a super-regenerative receiver272 having an output at a line 274 coupled to the bandpass filter whichprovides digital output to the bandpass filter 212. The bandpass filter212 includes a first stage 276 and a second stage 278 to provide adigital level output signal at a lead 280 which is supplied via anaveraging circuit 282 to an input pin P32 of the microcontroller 214.

The microcontroller 214 may have its mode of operation controlled by aprogramming or learning switch 300 coupled via a line 302 to the P25pin. A command switch 304 is coupled via a jumper 306 to a line 308 andultimately through a resistor to the input pin P22. A pin P21 sinkscurrent through a resistor 314 connected to a light emitting diode 316,causing the diode to light to indicate that the receiver is active. Themicrocontroller 214 has a 4 MHz crystal 328 connected to it to provideclock signals and includes an RS232 output port 332 that is coupled tothe pin P31. A switch 340 selects whether constant pressure ormonostable is to be selected as the output from output terminals P24 andP23 which are coupled to a transistor 350 which, when switched on, sinkscurrent through a coil 352 of a relay 354, causing the relay to close toprovide an actuating signal on a pair of leads 356 and 358 to anelectric motor.

It may be appreciated that the power supply 204 may receive power froman external transformer or other AC source through a jack 370 which isconnected to a pair of RJ uncoupling capacitors 372 and 374. The inputsignal is then set to a full-wave rectifier bridge 376 which provides anoutput current at a resistor 378. An 18-volt Zener diode 380 isconnected between ground and the resistor 378 and includes highfrequency bypass capacitor 382 connected in parallel with it. An8.2-volt Zener diode 384 is connected in back-biased configuration tothe resistor 378 to receive a signal therefrom to guarantee that atleast an 8.2-volt signal is fed to a resistor 390 causing an LED 392 tobe illuminated and also causing power to be supplied to a 5-volt 78LO5voltage regulator 396. The voltage regulator 396 supplies regulatedvoltage to an output line 398. Filtering capacitors 400 a, 400 b, 400 cand 400 d limit the fluctuations at the power supply.

The program code listing for the transmitter is set forth at pages A-1through A-19 and for the receiver at pages A-20 through A-51 of theattached appendix. Referring now to FIGS. 7A through 7C, the flow chartset forth therein describes the operation of the transmitter. A rollingcode is incremented by three in a step 500, followed by the rolling codebeing stored for the next transmission from the transmitter when thetransmitter button is pushed. The order of the binary digits in therolling code is inverted or mirrored in a step 504, following which in astep 506, the most significant digit is converted to zero effectivelytruncating the binary rolling code. The rolling code is then changed toa trinary code having values 0, 1 and 2 and the initial trinary rollingcode is set to 0. It may be appreciated that it is trinary code which isactually used to modify the radio frequency oscillator signal and thetrinary code is best seen in FIG. 6. It may be noted that the bit timingin FIG. 6 for a 0 is 1.5 milliseconds down time and 0.5 millisecond uptime, for a 1, 1 millisecond down and 1 millisecond up and for a 2, 0.5millisecond down and 1.5 milliseconds up. The up time is actually theactive time when carrier is being generated. The down time is inactivewhen the carrier is cut off. The codes-are assembled in two frames, eachof 20 trinary bits, with the first frame being identified by a 0.5millisecond sync bit and the second frame being identified by a 1.5millisecond sync bit.

In a step 510, the next highest power of 3 is subtracted from therolling code and a test is made in a step 512 to determine if the resultis equal to zero. If it is, the next most significant digit of thebinary rolling code is incremented in a step 514, following which flowis returned to the step 510. If the result is not greater than 0, thenext highest power of 3 is added to the rolling code in the step 516. Inthe step 518, another highest power of 3 is incremented and in a step520, a test is determined as to whether the rolling code is completed.If it is not, control is transferred back to step 510. If it has,control is transferred to step 522 to clear the bit counter. In a step524, the blank timer is tested to determine whether it is active or not.If it is not, a test is made in a step 526 to determine whether theblank time has expired. If the blank time has not expired, control istransferred to a step 528 in which the bit counter is incremented,following which control is transferred back to the decision step 524. Ifthe blank time has expired as measured in decision step 526, the blanktimer is stopped in a step 530 and the bit counter is incremented in astep 532. The bit counter is then tested for odd or even in a step 534.If the bit counter is not even, control is transferred to a step 536where the output bit of the bit counter divided by 2 is fixed. If thebit counter is even, the output bit counter divided by 2 is rolling in astep 538. The bit counter is tested to determine whether it is set toequal to 80 in a step 540. If it is, the blank timer is started in astep 542. If it is not, the bit counter is tested for whether it isequal to 40 in a step 544. If it is, the blank timer is tested and isstarted in a step 544. If the bit counter is not equal to 40, control istransferred back to step 522.

Referring now to FIGS. 8A through 8F and, in particular, to FIG. 8A, theoperation of the receiver is set forth therein. In a step 700, aninterrupt is detected and acted upon from the radio input pin. The timedifference between the last edge is determined and the radio inactivetimer is cleared in step 702. A determination is made as to whether thisis an active time or inactive time in a step 704, i.e., whether thesignal is being sent with carrier or not. If it is an inactive time,indicating the absence of carrier, control is transferred to a step 706to store the inactive time in the memory and the routine is exited in astep 708. In the event that it is an active time, the active time isstored in memory in a step 710 and the bit counter is tested in a step712. If the bit counter zero, control is transferred to a step 714, asmay best be seen in FIG. 8B and a test is made to determine whether theinactive time is between 20 milliseconds and 55 milliseconds. If it isnot, the bit counter is cleared as well as the rolling code register andthe fixed code register in step 716 and the routine is exited in step718.

In the event that the inactive time is between 20 milliseconds and 55milliseconds, a test is made in a step 720 to determine whether theactive time is greater than 1 millisecond, as shown in FIG. 8C. If it isnot, a test is made in a step 722 to determine whether the inactive timeis less than 0.35 millisecond. If it is, a frame 1 flag is set in a step728 identifying the incoming information as being associated with frame1 and the interrupt routine is exited in a step 730. In the event thatthe active time test in step 722 is not less than 0.35 millisecond, inthe step 724, the bit counter is cleared as well as the rolling coderegister and the fixed register and the return is exited in the step726. If the active time is greater than 1 millisecond as tested in step720, a test is made in a step 732 to determine whether the active timeis greater than 2.0 milliseconds. If it is not, the frame 2 flag is setin a step 734 and the routine is exited in step 730. If the active timeis greater than 2 milliseconds, the bit counter rolling code registerand fixed code register are cleared in step 724 and the routine isexited in step 726.

In the event that the bit counter test in step 712 indicates that thebit counter is not 0, control is transferred to step 736, as shown inFIG. 8A. Both the active and inactive periods are tested to determinewhether they are less than 4.5 milliseconds. If either is not less than4.5 milliseconds, the bit counter is cleared as well as the rolling coderegister and the fixed code registers. If both are equal to greater than4.5 milliseconds, the bit counter is incremented and the active time issubtracted from the inactive time in the step 738, as shown in FIG. 8D.In the step 740, the results of the subtraction are determined as towhether they are less than 0.38 milliseconds. If they are, the bit valueis set equal to zero in step 742 and control is transferred to adecision step 743. If the results are not less than 0.38 milliseconds, atest is made in a step 744 to determine if they difference between theactive time and inactive time is greater than 0.38 milliseconds andcontrol is then transferred to a step 746 setting the bit value equal to2. Both of the bit values being set in steps 742 and 746 relate to atranslation from the three-level trinary bits 0,1 and 2 to a binarynumber.

If the result of the step 744 is in the negative, the bit value is setequal to 1 in step 748. Control is then transferred to the step 743 totest whether the bit counter is set to an odd or an even number. If itis set to an odd number, control is transferred to a step 750 where thefixed code, indicative of the fact that the bit is an odd numbered bitin the frame sequence, rather an even numbered bit, which would implythat it is one of the interleaved rolling code bits, is multiplied bythree and then the bit value added in.

If the bit counter indicates that it is an odd number trinary bit beingprocessed, the existing rolling code registers are multiplied by threeand then the trinary bit value obtained from steps 742, 746 and 748 isadded in. Whether step 750 or 752 occurs, the bit counter value is thetested in the step 754, as shown in FIG. 8E. If the bit counter value isgreater than 21, the bit counter rolling code register and fixed coderegister are cleared in the step 758 and the routine is exited. If thebit counter value is less than 21, there is a return from the interruptsequence in a step 756. If the bit counter value is equal to 21,indicating that a sink bit plus trinary data bits have been received, atest is made in a step 760 to determine whether the sink bit wasindicative of a first or second frame, if it was indicative of a firstframe, the bit counter is cleared and set up is done for the secondframe following which there is a return from the routine in the step762. In the event that the second frame is indicated as being receivedby the decision of step 760, the two frames have their rollingcontributions added together to form the complete inverted rolling code.The rolling code is then inverted or mirrored to recover the rollingcode counter value in the step 764. A test is made in the step 766 todetermine whether the program mode has been set. If it has been set,control is transferred to a step 768 where the code is compared to thelast code received. If there is no match, as would be needed in order toget programming, then another code will be read until two successivecodes match or the program mode is terminated. In a step 770, the codesare tested such that the fixed codes are tested for a match with a fixedcode in non-volatile memory. If there is a match, the rolling portion isstored in the memory. If there is not, it is stored in the non-volatilememory. Control is then transferred to step 772, the program indicatoris switched off, the program mode is exited and there is a return fromthe interrupt. In the event that the test of step 766 indicates that theprogram mode has not been set, the program indicator is switched on in astep 774, as shown in FIG. 8F. The codes are tested to determine whetherthere is a match for the fixed portion of the code in the step 776. Ifthere is no match, the program indicator is switched off and the routineis exited in step 778. If there is a match, the counter which isindicative of the rolling code is tested to determine whether its valueis greater than the stored rolling code by a factor or difference ofless than 3,000 indicating an interval of 1,000 button pushes for thetransmitter. If it is not, a test is made in the step 786 to determinewhether the last transmission from the same transmitter is with arolling code that is two to four less than the reception and, if true,is the memory value minus the received rolling code counter valuegreater than 1,000. If it is, control is transferred to a step 782switching off the program indicator and setting the operation commandword causing a commanded signal to operate the garage door operator. Thereception time out timer is cleared and the counter value for therolling code is stored in non-volatile memory, following which theroutine is exited in the step 784. In the event that the difference isnot greater than 1,000, in step 786 there is an immediate return fromthe interrupt in the step 784. In the event that the counter test in thestep 780 is positive, steps 782 and 784 are then executed thereafter.

While there has been illustrated and described a particular embodimentof the present invention, it will be appreciated that numerous changesand modifications will occur to those skilled in the art, and it isintended in the appended claims to cover all those changes andmodifications which fall within the true spirit and scope of the presentinvention.

What is claimed is:
 1. A receiver for receiving an encrypted signal froma transmitter and for generating an actuation signal, comprising:receiving apparatus for receiving an encrypted radio frequency signal; ademodulator for demodulating the encrypted radio frequency signal into ademodulated encrypted signal; code separating apparatus forsynchronizing with and separating the demodulated encrypted signal intoa variable code portion and a fixed code portion; testing apparatus fortesting the fixed code portion and generating a portion receipt signalin response thereto; window testing apparatus for testing whether arolling code value is within a windowed range of rolling code values toavoid false triggering on a spurious rolling code having beentransmitted by a person not having permission to authorize generation ofthe actuator signal and producing a rolling code signal in responsethereto; a code updater for updating the variable code to an incrementedrolling code; and signal generating apparatus for producing an actuatorsignal to produce an actuator command in response to the verified fixedcode portion and the variable code.
 2. A receiver for receiving anencrypted signal from a transmitter and for generating an actuationsignal, comprising: a receiver for receiving an encrypted radiofrequency signal; a demodulator for demodulating the encrypted radiofrequency signal into a demodulated encrypted signal; a first comparatorfor comparing the encrypted signal to a first plurality of signal codevalues defining a valid signal window and in response thereto generatingan actuator signal; a second comparator for comparing the encryptedsignal to a second set of values and generating a security code signalin response thereto; a third comparator for comparing a second signaltransmission; and a fourth comparator responsive to said security codesignal for comparing a second successive transmission to said secondwindow values and producing an actuation signal in response thereto andan inhibitor for inhibiting actuation responsive to receipt of a signalhaving a value within a trailing signal window.
 3. A receiver forreceiving an encrypted signal from a transmitter for generating anactuation signal according to claim 2, further comprising apparatus fordetermining whether the second of the two successive codes is valid bycomparing the first received code to the second received code andproducing the actuation signal only when the first received code and thesecond received code are separated by a single rolling code increment.4. A receiver for receiving an encrypted signal from a transmitter andfor generating an actuation signal according to claim 2, furthercomprising a code signal generator for generating a fixed code signal inresponse to said encrypted radio frequency signal, said fixed codesignal being compared to a stored fixed code signal and enablingactuation only upon a match.
 5. A receiver for receiving an encryptedsignal from a transmitter and for generating an actuation signalaccording to claim 2, wherein said demodulated encrypted signalcomprises an interleaved fixed code portion and a variable code portionboth said fixed code and said variable code portions comprisingmulti-valued digits and each digit having at least 3 possible values. 6.A receiver for receiving an encrypted signal from a transmitter and forgenerating an actuation signal according to claim 5, further comprisinga converter for converting said multi-valued digits to binary digits. 7.A receiver for receiving an encrypted signal from a transmitter and forgenerating an actuation signal according to claim 5, further comprisinga converter for converting the demodulated encrypted signal into abinary fixed code signal and a binary variable code signal.
 8. Areceiver for receiving an encrypted radio frequency signal from atransmitter and for generating an actuation signal, comprising: areceiver for receiving an encrypted radio frequency signal; ademodulator for demodulating the encrypted radio frequency signal into ademodulated encrypted signal; a rolling code generator responsive to thedemodulated encrypted signal for producing a rolling code signal; aforward tester for testing whether the rolling code signal is within aforward rolling code window and generating an actuation authorization inresponse thereto; a backward tester for testing whether the rolling codeis within a backward rolling code window and inhibiting actuation of anactuator in response thereto; a resynchronization tester for testingwhether the rolling code is within a rolling code resynchronizationwindow and generating a resynchronization signal in response thereto; arolling code resynchronizer for resynchronization of a receiver rollingcode value in response to the resynchronization signal; and a controlsignal generator for generating a control signal in response to theactuation authorization for control or actuation of the actuator.
 9. Areceiver for receiving an encrypted radio frequency signal from atransmitter and for generating an actuation signal according to claim 8further comprising: an authenticity determiner for determining theauthenticity of the rolling code and the decrypted fixed code signal andgenerating a control signal in response thereto for control or actuationof an actuator.